The main function of analgesia is to alleviate pain or suppress its perception in a subject.
Pain can be defined in a variety of ways. One of them establishes pain as the perception by a subject of noxious stimuli which produces a withdrawal reaction by the subject. A variety of methods have long been available for the evaluation of subject's response to pain. In general, these experimental procedures must comply with several criteria in order to be of use. The effect to be measured must be unequivacally related to the pain experimented by the subject. The applied stimulus must produce a reproducible and measurable response, and ideally the response should be a single response. However, rarely do methods comply with the latter requirement.
It is recognized that there are at least two modes of pain known as pricking and burning pain. The sensation of pricking pain is one that reaches a sharp peak and subsides quickly while the one of burning pain has a slower onset, rises gradually, then plateaus but never reaches the same type of peak or intensity observed with pricking pain, and lasts for a longer period of time (Keith E.F., Amer. J. Pharm. 132:212-230 (1960)).
Pain has also been classified in accordance with other criteria (Pfeiffer, C.C. et al, Ann. N.Y. Acad. Sci. 51:21 (1948)). "Supain" is used to describe superficial pain and is similar to the previously referred to as pricking pain. It can easily be elicited by pricking the skin with a needle. The second type of pain is "deepain" which is of an aching character. An example of this type is a tooth ache. Lastly, another type of pain is "sympain" which is exemplified by migraine headaches and is typically felt in the vessels of the temporal region. This pain is relieved by ganglionic blockade. The levels at which pain may be blocked are the receptors themselves. Many agents which block pain by acting superficially do so at the receptor itself. A second way of acting is that where the agents act at the internuncial pool of the spinal cord by blocking pain or raising the threshold for synaptic transmission. Centrally, the thalamus of the brain may also be involved in suppressing pain, or the cortex of the brain may be obtunded. Some analgesic agents may suppress muscle and joint pain. An example of these agents is salicylic acid which relieves pain from deep structures in muscular joint areas.
In order to study the mechanism of action of the different available drugs as well as novel drugs which are constantly being developed there is a broad background of techniques from which to select. However, all the available techniques have some beneficial and some detrimental characteristics. In general, the techniques can be divided into five groups:
1) Chemical methods PA1 2) Electrical methods PA1 3) Mechanical methods PA1 4) Thermal methods PA1 5) Pharmacological methods
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